A temperature-responsive hygroscopic material, which absorbs and releases moisture in the air according to temperature variation, is conventionally used in desiccant type dehumidifiers and air conditioners. Even if the temperature-responsive hygroscopic material degrades its hygroscopicity by absorbing moisture, it can recover the hygroscopicity by releasing moisture under predetermined conditions. Therefore, the temperature-responsive hygroscopic material is repeatedly used as a recyclable hygroscopic material.
As an example of such a temperature-responsive hygroscopic material, Non-Patent Literature 1 discloses a composite silica gel in which a temperature sensitive polymer and mesoporous silica gel are composited. It is disclosed that the moisture adsorption amount of the composite silica gel decreases at a temperature higher than the phase transition temperature of the temperature sensitive polymer. The composite silica gel is produced by: immersing mesoporous silica in a mixture solution of monomers of the temperature sensitive polymer and a polymerization initiator; thereafter causing thermal polymerization.
Patent Literature 1 discloses: a membrane electrode assembly including a temperature-responsive layer in which a temperature-responsive material is chemically bonded with a porous layer in pores of the porous layer; and living radical polymerization as a method for producing the temperature-responsive layer. It is described that the temperature-responsive material is difficult to be infiltrated or retained in pores of the porous layer when the average pore diameter of the porous layer is less than 50 nm, since the pores are too small. Patent Literature 2 discloses a temperature sensitive adsorption material including porous silica whose outer surface (surface other than the inner surfaces of pores) is uniformly covered with a small amount of a temperature sensitive polymer, wherein the inlet diameters of pores of the porous silica can be controlled by temperature control.